It is previously known in bipolar transistors to provide an emitter system consisting of mutually parallel emitter fingers with emitter resistors, one for each emitter finger and generally provided outside the emitter region and generally in the prolongations of the respective emitter fingers. To a certain degree, this solves the problem of current increase due to local overheating of one of the emitter-base regions, which leads to thermal runaway. However, experience shows that even with such ballast resistors, local heating occurs, such that all the fingers heat more at the respective portions where they are connected to the ballast resistors than at other portions, and infrared photographs indicate rather that a collective heating between the collector fingers occurs at high loads, making the total efficiency less than optimal.
Solutions according to the concept as mentioned above are known, where emitter ballast resistors are situated within the semiconductor devices, instead of in the prolongations of elongate emitter fingers or linear arrays of emitter patches. Such solutions are known from U.S. Pat. Nos. 5,374,844 and 5,444,292. Between those solutions is common that resistive layers are arranged between emitter zones in the silicon and metal overlay emitter leads, such that current therebetween is submitted to a resistive path depthwise and sidewise, perpendicular to the emitter direction or emitter patch linear direction.